(i) Technical Field
The present invention relates to a toner for developing an electrostatic image that is used for developing an electrostatic latent image formed by an electrophotographic method or an electrostatic recording method with a developer, and a method for producing the same. The invention also relates to a developer for developing an electrostatic image and a method for forming an image that utilize the toner for developing an electrostatic image.
(ii) Related Art
It has been a major issue to improve dispersibility of a releasing agent in a toner. In the case where a releasing agent is dispersed insufficiently, aggregation in the toner or exposure on the toner surface occurs to cause such a problem as releasing agent offset caused by the releasing agent remaining on a photoreceptor, which might adversely affect image quality particularly on a transparent image forming material, such as an OHP sheet.
A vinyl polymer has been widely used as a binder resin of a toner. Upon using a vinyl polymer having a high molecular weight having a high softening temperature, a fixing roller should be set at a high temperature for obtaining a fixed image excellent in glossiness, which is contrary to energy saving.
On the other hand, a polyester resin has high flexibility as compared to a vinyl polymer owing to a rigid aromatic ring contained in the chain, and thus the molecular weight can be set lower for obtaining the same mechanical strength. Furthermore, a polyester resin has such an advantage that it can be easily designed as a resin for being fixed at a low temperature from the standpoint of entanglement of molecular chains and limited molecular weight as compared to a vinyl binder resin, and thus a polyester resin is often used as a binder resin of a toner.
A polycondensation resin and a condensation compound are generally synthesized by using a metallic compound as a catalyst from the standpoint of the high reactivity thereof and prevention of coloration.
An ordinary condensation or polycondensation reaction generally proceeds at a high temperature exceeding 200° C. and highly reduced pressure under agitation with a large driving force for a prolonged period of time of 10 hours or more, which brings about consumption of a large amount of energy. Furthermore, a large amount of spending is often required for the reaction equipments satisfying the necessary durability.
Investigations relating to a method for producing a polyester resin at a low temperature have been reported in recent years.